Formulations and blends comprising polycarbonates are known to degrade upon prolonged exposure to sunlight or other forms of light. One of the effects observed is yellowing of the polycarbonate blend/formulation. This problem has been alleviated by the use of light absorbers or light blockers in the polycarbonate blends. Thus ultraviolet light absorbers (UVA) are known to be used in polycarbonate formulations to protect these formulations from degradation due to exposure to different forms of light.
Use of Hindered Amine Light Stabilizers (HALS) to stabilize polyolefins has been known. Commercially used HALS have been based mainly on 2,2,6,6-tetramethyl piperidine, except for those based on piperazinones. U.S. Pat. Nos. 4,190,571; 4,292,240; 4,480,092; and 5,071,981 disclose some of the piperazinone based HALS. In addition while U.S. Pat. Nos. 3,919,234, 3,920,659 and 4,208,522 disclose some piperazine dione based HALS.
The use of HALS in polycarbonates is not very common. The stabilizing effect of HALS on polycarbonate formulations/blends has been described as "minor", see for example Thompson and Klemchuk, in Polymer Durability; R. L. Clough et al., ACS Advances in Chemistry 249, 1995, pp 303-317. The use of UVA in polycarbonate formulations has provided a way to retard the rate of degradation of polycarbonates as a result of exposure to light. This method however does not offer a complete protection of polycarbonate formulations from light induced degradation or discoloration. There is thus a continued need for a method or formulation that will help protect polycarbonate based formulations/blends from degradation or discoloration due to exposure to light.
It has been surprisingly found that use of the piperazinone and piperazine dione based HALS, and the benzotriazole, benzophenone, triazine, oxanilide, and cyanoacrylate based UVA in polycarbonate compositions, exhibits a synergistic effect in protecting the polycarbonate compositions from light induced degradation. This synergistic effect increases the photostability of polycarbonate compositions thereby slowing the yellowing and degradation of polycarbonate compositions.